Evaluation of damage-induced permeability in the Bure site using a three dimensional adaptive continuum / dis-continuum code

ITASCA has developed and used Dis-continuum models to study the damage of rock, by building particle assemblies and checking the breakage of bonds under stress. However, such models have been limited in size by the very large number of particles needed to model even a comparatively small volume of rock. In fact, not all zones in a model are necessarily submitted to large strains and show contact breaks. A large part of most models never experiences large strains, and does not require the accurate description of large-strain / damage / post peak behaviour afforded by a Dis-continuum model. Thus, very often a large model can be separated into a strongly strained core area to be represented by a Dis-continuum, and a peripheral area where Continuum zones would be adequate. From this observation, members of the Itasca group have developed a coupled, three-dimensional, continuum/dis-continuum modelling approach. The new approach, termed Adaptive Continuum/ Dis-continuum Code (AC/DC), is based on the use of a periodic Dis-continuum base brick, for which more or less simplified continuum equivalents are derived. The AC/DC code can dynamically select the appropriate brick type to be used in various parts of the model, depending on the level of deformation in each part (i.e. switch locally from a continuum to a granular description based on selected criteria). In this paper, we apply the new approach to an excavation performed in the Bure site. The modelling aims at evaluating the damage-induced variation of permeability at the periphery of a shaft, during the first two years of its opening. In a first part, we present the concepts developed to build a numerically efficient mixed model while minimizing the effect of continuum/dis-continuum boundaries. We also describe the particle model developed to reproduce the short and long term behaviours of the Callovo-Oxfordian argilite. In a second part, we show the numerical model built to simulate the shaft excavation. We summarize the calibration process used to define the short and long term parameters, as well as the assumptions considered. Finally, we present an interpretation in terms of crack-induced permeability at the periphery of the shaft. A (crack transmissivity vs. opening) relation is first calibrated using laboratory results, and then applied to the crack network generated during the mechanical simulation. (authors)